1. Field of the Invention
The present invention relates generally to position detecting apparatus and a position pointing device which can be used therewith, by making use of electromagnetic waves.
2. Description of the Related Art
The applicant's assingnee has proposed, in European Patent Publication No. 0259894, a position detecting apparatus which transmits and receives an electromagnetic wave between a sensor unit of a position detection section (here referred to as a tablet) and a position pointing device to obtain a coordinate value of a position on the tablet pointed to by the position pointing device. European Patent Publication No. 0259894 is hereby incorporated by reference. European Patent Publication No. 0259894 is a related case of U.S. Pat. No. 4,878,553. U.S. Pat. No. 5,028,745 issued from a Divisional application of U.S. Pat. No. 4,878,553.
This apparatus is equipped with a plurality of loop coils placed within the tablet and arranged in parallel to each other along a position detecting direction. The plurality of loop coils are selected in turn to transmit an electromagnetic wave to a resonance circuit in the pointer and to receive an electromagnetic wave re-transmitted from a resonance circuit. The loop coil which develops the strongest reception signal enables the coordinate position of the position pointing device to be determined.
The position detecting apparatus of the type described faces a demand for ability to enter various kinds of additional information in addition to the coordinate values of the pointed position, such as information which indicates that the pointing means (such as a pen) has been set to a position for pointing a definite position (this state will be referred to as a `pen-down` state, hereinafter), information concerning the type of the pointing means, e.g., a pen, a cursor, or the like, as well as a demand for inputting, together with the coordinate values, information for continuously varying values of parameters other than the width of a line and hue or density (brightness) of the position or area appointed by the pointing device.
To cope with such demands, the position pointing device disclosed in Japanese Patent Laid-Open No. 63-70326 employs a manual switch which selectively connects an additional capacitor so as slightly to vary the resonance frequency. The slight variation in the resonance frequency is detected as a variation in the phase angle so as to be used as information indicative of the pen-down state or the type of pointing means.
However, the detectable range of the phase angle in the apparatus of the prior application is limited to from about -60 to about +60 degrees. Furthermore, a certain tolerance for the phase angle to be detected has to be set in order to accommodate any change in the inductance of the coil and capacitance of the capacitor of the resonance circuit caused by a change in the ambient temperature. For these reasons, the number of categories of information that can be entered is undesirably limited. Especially, in case of allowing the continuous variation in resonance frequency corresponding to the pen pressure, no information other than the pen pressure could be inputted.
The same applicant also proposed in Japanese Patent Application No. 63-308712 (Japanese Patent Laid-Open No. 2-155019), an input device which can input plural pen pressure, or a combination of pen pressure information and other information by using a position pointing device equipped with a resonance circuit which is changeable to plural realms of predetermined frequencies corresponding to the ON/OFF state of a side switch while enabling to change the resonant frequency continuously in accordance with the pen pressure.
In the aforesaid apparatus of the prior application a realm of phase angle in relation to the fixed frequency was assigned to divided frequency realm responding to the ON/OFF state of the side switch, so that the range for detecting pen pressure became small, as well as needing two different sets of correction indexes due to the differences in the phase angle for the pen pressure whether the switch is ON or OFF.
Furthermore, the applicants in Japanese Patent Laid Open No. 5-233126 and Japanese Patent Laid-Open No. 2-155019, proposed an apparatus wherein the center of a resonance frequency varies to one predetermined frequency in accordance with the switch operation, making use of a position pointing device having a resonance circuit resonance frequency changes continuously centered on one of at least two said plural frequencies corresponding to pen pressure, transmitting plural frequency electromagnetic waves toward said position pointing device, then receiving an electromagnetic wave retransmitted from the resonance circuit, to detect the coordinate value of the pointed position and the phase angle, enabling plural pen pressure or pen pressure information and other information to be input by identifying the resonance frequency to one of said plural predetermined frequencies, and enabling pen pressure information toward the phase angle to be corrected without reducing range pen pressure detection range.
The aforementioned application, however, had a problem of needing adjustments for each of the plural resonance frequencies as well as needing a wide range of frequencies in order to avoid overlapping when pen pressure changes.